Many different types of adjustable fin systems are known particularly in relation to surfboards. One such system is described in U.S. Pat. No. 4,044,416 (Brewer et al) which discloses a fin holder set in the underside of a surfboard defining an elongate channel for receiving the base of a fin for lengthwise adjustment of the fin relative to the surfboard. A clamp is provided which is slidable lengthwise in the channel and is engagable with the fin for releasably retaining the fin relative to the surfboard.
A further system is described in U.S. Pat. No. 5,997,376 (Block et al) which discloses a fin mounting system for a surfboard including a box which is embedded within a surfboard by producing a through cavity within the surfboard, placing the box within the through cavity and then pouring a hardenable resin between the box and the cavity to fix the same in the surfboard. A separate fin is then inserted into the box. The fin is adjustable longitudinally and can be fixed into a number of different positions by way of a fastener comprising a conventional nut and bolt where the nut is embedded into the fin.
Another system is described in U.S. Pat. No. 5,215,488 (Bailey) which teaches the insertion of a mounting box defining an elongated channel into the hull of a watercraft and a fin adjustably mounted in the mounting box. A locking device is provided for adjustably locking the fin at different positions along the channel. The locking device includes a flat parallelogram shaped locking nut with a rotational biasing spring which are supported by the fin, with the nut receivable within an internal recessed lateral groove formed in the elongated channel.
Further fin mounting systems are described in U.S. Pat. No. 4,421,492 (Leva) and U.S. Pat. No. 5,934,963 (Frizzell).
While all of the abovementioned fin mounting systems provide different means for moving a fin longitudinally relative to the hull of a watercraft, they suffer from one or more of the following problems. The mounting boxes in many cases are provided with openings for receiving a fin which are exposed to water flow, or are provided with clamps or other locking devices which are directly exposed to water flow thus adversely affecting the hydrodynamics of the associated water craft. Some of the systems also require the use of screws or nuts which require tools such as screwdrivers or spanners in order to allow adjustment of the fin. These systems are particularly disadvantageous if it is required to change the position of the fin while in the water where typically, at least in the case of a surfboard, the user of the craft would not be in possession of such tools. Further, systems which require screws, nuts and/or springs are often subject to rapid degradation by action of water, and in particular salt water. Another problem is that the typical forces created in use of the fins act in a manner to disengage the fin in some of the prior at mounting systems.